Copper, gold and other mined ores are blasted or crushed into small chunks and placed directly into large heaps where the ore can be irrigated with a leach solution of, e.g, sodium cyanide (NaCN), potassium cyanide (KCN), or sulphuric acid (H2SO4), applied to the pile. The solution percolates through the heap, bonds to particles of metals such as gold or copper, and the leach solution is then captured (e.g., by an impermeable plastic or clay-lined leach pad at the bottom of the pile) and processed into pure metal. The leach process through a pile typically takes several weeks. Once the metals are removed, the leach solution is recycled and used again on the pile to leach more metal.
In the past, large irrigation sprinklers have been used to deliver the leach solution to the pile. This practice is rarely seen today because of the tremendous amount of water loss (e.g., evaporation and runoff) with this method.
More traditionally, the leaching solution is delivered to the piles using drip irrigation, i.e., nonporous tubing with drip emitters molded into the tubing at spaced intervals (most often at 28 inch intervals), the irrigation system being laid out across the surface of the pile. Based on their flow characteristics and periodic placement along the line, drip emitters cause a phenomenon known as channeling in the ore. Channeling occurs where a large amount of solution is dripped in one spot over time. This spot quickly saturates and the solution then channels into a relatively narrow stream, traveling quickly down through the pile as the solution seeks the path of least resistance to the bottom of the pile. Channeling produces a very uneven distribution of the leaching solution in the pile, with some areas extremely wet and others dry. Gold and copper are only leached from ore that makes direct and extended contact with the leach solution, and therefore in piles using drip emitters there are areas between the channels that do not get adequate leaching and metal removal.
Still further, because drip irrigation applies the solution to the surface of the pile, and because a large volume of solution is dripped onto a select spot, drip emitters cause pooling and ponding of the leach solution on the top surface of the ore pile. Both sodium cyanide and sulphuric acid can be toxic to wild life. In various jurisdictions, there is a major push for all leach lines to be buried because of the negative environmental impact of surface drip irrigation. While burying drip irrigation is possible, when buried the emitters tend to clog, further reducing the efficiency of the leaching process and greatly increasing the amount of labor required to keep them in operation.
FIG. 1 illustrates the problems encountered with burying a standard drip line emitter below the surface of the pile. FIG. 1 is a schematic cross section of a leach pile 5 showing a drip line 7 buried 6 to 18 inches below the top surface 6 of the pile. The emitters 8 are shown in cross section, disposed every 28 inches across a section of the pile. For ease of illustration, only a small section of the pile is shown, it being understood that a typical pile may range from 300 by 300 feet in cross sectional area, increasing up to 1000 by 1000 feet in cross sectional area, with a starting height of 40 feet, and increasing over time to 1000 or more feet high. Thus, these piles of ore are truly enormous and their very large scale must be taken into consideration in understanding the problems addressed by the present invention.
On the left hand side of FIG. 1, a standard drip line 7 is shown buried beneath the surface of a leach pile having a relatively low clay content (i.e., less compacted than a high clay content pile). Although the leach solution 9 can quickly saturate the area below each of the individual emitters 8, spanning out in the process, there are still large areas between the emitters where the amount of leaching solution is deficient, reducing the efficiency of the process. Still further, when leach emitters are buried, the emitters quickly clog with particles of ore, sand or soil, further reducing the efficiency of the leaching process and greatly increasing the amount of labor required to keep them in operation.
On the right hand side of FIG. 1, a standard drip line 7 is shown buried beneath the surface of a leach pile having a high clay content, or otherwise compacted. Here the problems are even greater. The point source delivery of the solution is not absorbed quickly by the pile due to its high density, and so the solution flows along the drip line 7, concentrates, and rises to the surface 6 of the pile forming a pool 9; it also discharges from the side 11 of the pile. The pooling of leach solution on the surface leads to evaporation losses and potential injury to wildlife.
As a result of these multiple problems with buried drip lines, the vast majority of users continue to lay the drip line emitters on the top surface of the pile. It is both easier and cheaper, avoiding the expense of burying the drip line, avoiding the emitters being crushed or damaged by the process of burying them under the ore, and reducing the likelihood that the emitters will become clogged by the particles in the ore pile.
There has long been a need for improvements in the current methods for delivering a leaching solution to a pile of mining ore in an efficient and cost effective manner. Despite such long felt need, there have been little changes in the process over the past decades.